1. Field of the Invention
This invention relates to compositions useful as adhesives and more particularly to the preparation of epoxy-based adhesive compositions with improved impact resistance and/or improved adhesion to oily metal substrates.
2. Brief Discussion of the Related Art
Numerous compositions and processes are described in the art for making and using a wide variety of epoxy-based compositions and other resins and additives in an effort to improve the expansion, impact resistance and other key properties of adhesives useful in adhering, filling and making composite structures. For example, patents which describe components for the formulation of adhesive compositions and the use of such compositions to adhere various substrates to each other and to provide structural reinforcement include U.S. Pat. Nos. 5,290,857, 5,686,509, 5,334,654, 6,015,865, 5,278,257, 6,884,854, and 6,776,869 and U.S. Patent Application Publication No. 2005-0022929.
For advanced structural adhesive operations such as vehicle assembly, adhesives are required which provide broad cure schedules, stiffness improvement, weld reduction and energy/management. In particular, it would be highly desirable to develop heat-curable structural adhesives that not only have these characteristics but also are capable of forming strong bonds to metal surfaces contaminated with oily substances (especially surfaces comprised of cold rolled steel, which has the advantage of being significantly less expensive than other types of metals that could be used in vehicle construction).
In particular, it is known that the use of core-shell rubber particles that are stably dispersed in an epoxy resin matrix and that are nano-sized (e.g., about 25 to about 200 nm) are capable of improving the impact properties of epoxy-based adhesives. However, such materials tend to interfere with the ability of the cured adhesive to bond strongly with an oil-contaminated metal surface.